This invention relates to a developing apparatus for an image forming machine, and more particularly to a developing apparatus used with a dry type electrostatic copying machine applying a toner as a developing agent.
A large variety of developing apparatus have generally been proposed for a dry type electrostatic copying machine. Most of the proposed developing apparatuses which are practically applied are of the type in which a magnetic roller is set in the body of the dry type electrostatic copying machine; a magnetic toner, that is, the so-called one-component toner or a 2-phase mixture of a carrier (magnetized powder) and toner held in a toner feeder is adsorbed to the magnetic roller; the adsorbed mass acts as a magnetic brush; and the magnetic brush is made to slide over a sensitized layer, thereby transferring a required quantity of a magnetic toner or toner onto the sensitized layer. However, a toner feeding system whose main component is formed of the above-mentioned magnetic roller has the drawbacks that the required magnetic roller is expensive, presenting difficulties in reducing the manufacturing cost of a dry type electrostatic copying machine as a whole; a space between the peripheral surface of the magnetic roller and a photosensitive body has to be adjusted minutely, often with a precision of the order of about 0.1 mm; if an attempt is made to meet this space precision requirement, then a dry type electrostatic copying machine would involve a complicated arrangement; and the one-component toner in particular noticeably tends to demand a more rigid requirement for the above-mentioned space precision.
On the other hand, another proposed developing apparatus in which a toner is supplied by means of a carrier has the drawbacks that the carrier itself has a limited life, namely, has to be replaced by a fresh one, each time an impression is made on, for example, 10,000 to 20,000 copy sheets; and the replacement has to be undertaken, for example, by a maintenance service man, the often consumes a great deal of work and time.
Recently, therefore, a new developing apparatus has been proposed which uses an inexpensive fur brush roller. With one type of a fur brush roller developing apparatus (FIG. 1), long hairs 12 are fitted to the peripheral surface of the fur brush roller 10. The long hairs 12 stroke the surface of a photosensitive layer 14 to adsorb a toner 16 to the surface, thereby carrying out development.
With another type of the fur brush roller developing apparatus (FIG. 2), short hairs 20 are fitted to the peripheral surface of a fur brush roller 18, whose core 22 is made of soft material. Those portions of the peripheral surface of the fur brush roller 18 which contact the surface of the photosensitive layer 14 are made to flex to adsorb the toner 16 to the surface of the photosensitive layer 14, thereby carrying out development.
However, the first mentioned prior art fur brush roller developing apparatus has the drawbacks that the long hairs 12 attached to the fur brush rollers 10 are thrown flat during long application periods, reducing contact between the long hairs 12 and the surface of the photosensitive layer 14, and deteriorating the quality of a printed impression due to the reduced distinctiveness and blurring of the impression; the long hairs 12 have an increased electric resistance, preventing bias voltage for fog-free development from being fully impressed on the surface of the photosensitive layer 14, resulting in the failure to give full play to the fog-suppressing effect of the bias voltage; the smoky scattering noncharged excess toner by the rotation of the fur brush roller 10 leads noticeable fogging; and consequently a separate fog-preventing roller 24 has to be additionally provided, thereby complicating the arrangement of the first mentioned fur brush roller developing apparatus.
For the effective suppression of fogging by the development bias voltage in the latter type of the conventional fur brush roller developing apparatus, the particles of the toner 16 held between the short hairs 20 should be fully charged by friction between the toner particles 16 and hairs 20, and further a prescribed amount of the toner 16 should be supplied. Where sufficient friction does not take place between the short hairs 20 and toner particles 16, and further an excess amount of the toner 16 is supplied, then a certain portion of the toner 16 fails to contact the short hairs 20, causing an increased amount of toner particles 16 to remain noncharged. The noncharged toner particles 16 lead to fogging. Conversely, an insufficient amount of toner particles 16 results in a decline of the density of an impressed pattern. Consequently, as shown in FIG. 2, a toner charging roller 28 and toner feeding roller 30 have to be additionally provided, complicating the arrangement of the latter type of prior art fur brush roller developing apparatus. Moreover, the required simultaneous rotation of the respective rollers 18, 28, 30 demands the provision of a powerful motor, unavoidably increasing the cost of the developing apparatus.